Fix uninitialised use in mips_split_move

Fix uninitialised use in mips_split_move

While testing the fix for PR91068, I hit an rtl checking failure
while building newlib.  mips_split_move was decomposing an address that
happened to be symbolic and then tried to access the REGNO of the base
register field, which wasn't initialised but which by chance pointed to
valid memory.

2019-07-07  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/mips/mips.c (mips_split_move): Zero-initialize addr
	and check whether addr.reg is nonnull before using it.

From-SVN: r273174